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Tankless Water Heater Sizing Calculator
Add up what runs at the same time, pick how cold your water comes in, and see the flow rate, gas BTU or electric kW a tankless water heater needs for your home.
How tankless sizing works
- Flow rate: add the gallons per minute (GPM) of everything that runs at the same time.
- Temperature rise: subtract your winter incoming water temperature from your hot water setting.
- Pick a unit that can deliver that flow at that rise. The calculator turns it into gas BTU or electric kW.
Step 1: what runs at the same time
A tankless water heater heats water only as it flows. It never runs out, but it can only heat so many gallons per minute. So the question is not how much hot water you use in a day. It is how much you use at the same moment. Count what could realistically run at once on your busiest morning.
| Fixture | Typical flow |
|---|---|
| Shower | About 2.0 GPM (new showerheads are limited to 2.5 GPM) |
| Bathroom faucet | About 1.0 GPM |
| Kitchen faucet | About 1.5 GPM |
| Dishwasher | About 1.5 GPM |
| Clothes washer | About 2.0 GPM |
| Bathtub filling | About 4.0 GPM |
Check your own showerheads and faucets if you can. The flow rate is often printed on the fixture or in its manual, and low-flow fixtures make a big difference to the size you need.
Step 2: your temperature rise
The colder the water coming into the house, the harder a tankless heater has to work. In York County and much of the Mid-Atlantic, water from the main is often in the 40s in winter. In the South it can stay near 60°F. Always size for winter, because that is when a heater that is too small lets you down.
Most homes set hot water to about 120°F. With water coming in at 45°F, the heater has to raise it 75 degrees. A unit that delivers 8 GPM at a 35-degree rise in summer may deliver well under half of that at a 75-degree rise. That is the most common reason people are disappointed by a tankless heater.
Step 3: gas BTU or electric kW
The calculator turns flow and temperature rise into the size of heater you need. For gas, it uses the standard formula of GPM × 500 × temperature rise, divided by about 95% efficiency. For electric, it converts the same heat into kilowatts and amps at 240 volts.
- Gas or propane: most whole-house units are rated up to 199,000 BTU per hour. If you need more, two units can be linked together.
- Electric: whole-house electric units often need 100 amps or more on their own. Many older homes do not have that much spare capacity in the panel.
In cold-water areas like ours, gas usually makes more sense for a whole house. Electric tankless works best for a single bathroom, a small home, or as a point-of-use heater under a sink.
Before you buy
- A gas tankless unit may need a larger gas line than your old tank. Our gas line repair service can check the supply.
- Venting is different from a tank heater and must match the unit.
- Hard water builds scale inside the heat exchanger, so plan for descaling about once a year.
- If your home has a lot of short hot water uses, ask about a unit with a small buffer tank or a recirculation option.
Still deciding between tank and tankless? Our water heater size calculator sizes a tank by your busiest hour of use. For install or repair help, see our tankless water heater service and water heater repair service. You can find all our free plumbing tools in one place.
Tankless Sizing FAQs
What size tankless water heater do I need for a family of 4?
It depends on what runs at the same time and how cold your water is, not on the number of people. Two showers and a kitchen faucet at once need about 5.5 gallons per minute. In a cold-water area like Pennsylvania, that usually means a large gas unit. Enter your own fixtures in the calculator above to see your number.
What does temperature rise mean?
Temperature rise is how many degrees the heater has to warm the water. It is your hot water setting minus the temperature of the water coming into the house. Water entering at 45°F and heated to 120°F needs a 75°F rise. The bigger the rise, the less flow a tankless heater can deliver.
Why does a tankless heater give less hot water in winter?
Incoming water is colder in winter, so the heater has to raise it more degrees. A unit that easily runs two showers in summer may only manage one and a half in January. That is why the calculator asks for your winter water temperature.
Is gas or electric tankless better?
Gas units deliver much more hot water per unit, which suits whole-house use in colder areas. Electric units are simpler to vent but need a lot of power: whole-house models often need 100 amps or more on their own. The calculator shows both so you can compare.
Can I keep my old gas line and vent?
Often not. Gas tankless units can need a larger gas line and a different vent than a tank heater. A plumber should check the gas supply, venting and water connections before you buy.
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